This invention relates to automatic sawmills, and particularly to automatic sawmills that provide multiple, simultaneous cuts on logs optimally prepositioned for maximum lumber yield.
In view of increased competition, both foreign and domestic, the modern lumber industry has had to face a new range of problems in achieving economic production. Because of the increased cost of logs, their limited availability and generally smaller sizes, it is essential to obtain as much lumber value from each log as possible. Increasing equipment and labor costs have also made it necessary to obtain that lumber efficiently and economically In addition, it has become important to limit waste, both to increase yield and for environmental reasons. These factors then make it essential to modernize the lumber-producing art.
It has become standard practice to provide some degree of automation in a sawmill whereby human intervention is minimized. Thus, automatic means have been sought that will position longitudinal cuts through a log so as to obtain the maximum amount and value of lumber. An optical system employing lights or lasers and a video camera has been used to determine the size and shape of a log, with the log then being positioned relative to one or more saws so as to provide cuts that will yield the maximum.
The typical log will have some taper and curvature, and if such a log is not optimally oriented relative to a saw, the maximum yield of lumber value cannot be obtained. Previous systems such as disc U.S. Pat. No. 3,960,041 issued June 1, 1976 to Warren et al, have used a "backstanding" method in which all cuts are made parallel to one side of the log. Alternatively, it has sometimes been the practice to rotate each log into a desired position in a manner that introduces delay in production. Systems according to the prior art have often further required a sideways motion of the log after it has been oriented and optimally rotated and it would be advantageous to provide means for positioning and/or rotating a log that introduce minimum delay.
It has been customary to measure a log or cant while in motion, which, together with mechanical inaccuracy, leads to measurement errors. Scanning systems according to the prior art have often been somewhat coarse, not taking into account the presence of knots and indentations that could yield defective lumber. It would be useful, therefore, to employ an accurate scanning process, taking place with a log or cant in a fixed position, and one that measures an extended profile of the log or cant.
When a log has been optimally positioned for sawing, it is then essential that every cut be made as accurately as possible. The close dimensional tolerances necessary to obtain as much lumber and value from a log as predicted from its dimensions are difficult to meet because of the construction and orientation of the saws, and heat generated in the sawing process due in part to sawdust accumulation. It would be useful to provide means for improving sawing accuracy to achieve predictable output.
In prior systems that have made a single initial cut on a log, error and delay can arise from the manner of handling log pieces after the initial cut. The first pieces may fall onto slat beds or another conveyor that will carry them to next saws in line, and manual rotation of a piece may be required. The lack of positive control permits errors and delays in positioning for the next cut. In addition, a slat bed or the like is not perfectly horizontal over its length making it difficult to obtain a smooth, uniform cut upon a piece carried thereby.
From a slat bed or chain transfer conveyor, each piece may undergo a series of parallel cuts, and it may be necessary to reposition either the saw or the piece after each cut in order to make the next one. During the time of movement, no lumber production occurs. One solution to this problem has been to provide a sequential line of saws, requiring substantial space. Multiple vertical band mills can also be used in which multiple saws operate on the piece at the same time. Band mills of this type would position saws both above and below the piece being cut, which results in an expensive installation and precludes access for maintenance during saw operation. Sawdust accumulation is also a problem.
After longitudinal sawing of a log, waste slabs must be removed. In order to achieve the yield of lumber expected, continued accuracy in the handling of the cants is required, and the greater the number of cants involved, the more desirable it is to continue the automated procedure. In particular, cants are desirably rescanned to permit accurate placement of subsequent cuts. In the prior art, the cants have been scanned while in motion. However, that measurement does not provide a complete profile of the cant, and log movement introduces inaccuracies. It would again be useful, therefore, to scan while the cant is not moving, and also to obtain the complete cant profile.